This invention relates to a neutral point clamped electric power conversion apparatus, and in particular, it relates to a free wheel diode for use therefor.
Neutral point clamped electric power conversion apparatuses have been disclosed in Japanese Patent Applications Nos. 2-262827, 2-131370 and 1-198280, in which they comprise: first, second, third and fourth switching elements connected in series, with a series circuit of clamping diodes connected in reverse parallel between a connection node of the first and the second switching elements and another connection node of the third and the fourth switching elements; and a free wheel diode connected in reverse parallel to each one of the foregoing switching elements.
It has been confirmed through application of the above-mentioned prior art in actual neutral point clamped electric power conversion apparatus that there arises a new problem in the form of an imbalance in power dissipation or losses among the respective free wheel diodes. That is, if the same cooling design is applied to all of respective free wheel diodes, without taking into account their specific requirements for cooling, such an imbalance power losses causes a difference in the rate of degradation of these elements due to the heat cycle, which in turn causes an imbalance in the lifetimes of the outer diodes compared that of the inner diodes. This phenomenon eventually doubles the frequency of part replacement. Further, in more serious cases, the temperature of any free wheel diode in which a large loss occurs will increase until it breaks down.